Mary E Wilson and Lin H Chen

We live in a world of moving parts, fragmented, altered landscapes, and vast networks of connections. Human travel today is unprecedented in volume, reach, and speed (Wilson, 1995a). Spatial mobility for the average person increased more than 1000-fold between 1800 and 2000 (Cliff and Haggett, 2004; see Figure 1.1). Two million people cross international borders each day (WTO, 2006a). Movement of humans may be the result of planned travel - for business, tourism, education or research, missionary or volunteer work, visiting friends or family, or military purposes - but abrupt displacement of populations can also be a consequence of war, natural disasters, economic, political, or environmental events (Myers, 2001). Human activities have led to unprecedented movement or displacement of other species through intent (e.g. trade) or inadvertence (Smolinski etal., 2003). The conveyances of travel, such as ships and airplanes, can become places for transmission of infectious diseases. The roads and railroad tracks built to carry people and products from one location to another can also fragment habitats and can lead to decreased biodiversity. Table 1.1 provides examples of infectious disease dissemination associated with human travel.

Infectious diseases in people, plants, and animals are dynamic and are influenced by biological, environmental, socioeconomic, political, demographic, and genetic factors (Wilson et al., 1994; Wilson, 1995b). The abundance and variety of microbial life and its ongoing evolution mean that we will continue to encounter new infections or altered expressions of old ones in the foreseeable future. Among the characteristics of the global population today that favor the appearance and spread of infections - size, density, location, vulnerability, inequalities, and mobility - the latter will be the focus of this chapter.

Why does travel matter in the epidemiology of infectious diseases? Although many microbes that cause infectious diseases are found throughout the world, many others have a focal distribution because of the need for specific geoclimatic conditions or a particular intermediate or reservoir host, poor sanitation or control efforts, or other factors. Even for microbes that are globally distributed, risk of

Sfiah Scale Contagious Diseases
Figure 1.1 Spatial mobility: increased spatial mobility of the population of France, changes over a 200-year period, 1800-2000. The vertical scale is logarithmic, showing that growth in average travel distance has increased exponentially over time. Based on Cliff and Haggett (2004).

exposure may vary greatly from one geographic area to another (Freedman et al, 2006). Humans are interactive biological units; when they travel they can pick up pathogenic microbes that may make them sick (Wilson, 2003a). Whether they get sick or not, they may also carry pathogens or microbial genetic material (including resistance genes and virulence factors) in or on their bodies to a new location. Depending on the type of organism, its transmission mechanisms, and immunity of contacts, travelers may be able to introduce a pathogen into a new population.

Humans are social creatures who carry with them their traditions, customs, practices, dress codes, and values that may be observed and imitated or scorned and rejected by the people they visit. Travel typically is not just an origin and destination, but also includes stops and interactions along the way. Travelers have contact not only with local populations in areas they visit, but also with other travelers - who may reflect a wide range of geographic origins and carry their own microbiological baggage and customs that influence spread.

When people travel, they may engage in activities that they might not undertake at home. The experience in a new environment may lead them to have sexual contacts with new partners, try new foods or types of preparations, engage in risky water and other sports activities, receive injections or tattoos at local facilities, or handle or come into contact with animals. Many of these activities can put them at risk for infections that did not exist in their home environment.

Pathogen

Site of origin

Mode of spread

Consequences

Reference

HIV

Sub-Saharan

Truck routes in

14.5 million infected outside site of origin; approximately

UNAIDS,

Africa

Africa; air travel

40 million people lived with HIV worldwide at the end of 2005, 4 million became newly infected, and 2.8 million died.

2006

Measles

Asia (China,

Air travel

Outbreaks associated with international adoption have occurred.

CDC, 2004a,

India), Europe

An 11-year-old North Carolina resident traveled home from the United Kingdom via New York and Connecticut and transmitted measles to an infant contact; transmission to multiple states and countries was possible due to infectious period during flights. A 17-year-old adolescent who contracted measles in Bucharest, Romania, initiated an outbreak in Indiana, with 34 confirmed cases. An outbreak occurred in the Greater Boston area since May 2006, following an index case that had arrived recently from India.

2004b, 2005a, 2005b; Parker et al., 2006; Massachusetts DPH, 2006

Poliomyelitis

Six endemic

Migration;

Between 2002 and 2005, wild poliovirus spread to 21 previously

CDC, 2005c,

countries

air travel

polio-free countries; introduction led to sustained transmission

2006a, 2006b

(Afghanistan,

in 13 countries.

Egypt, India,

Niger, Nigeria,

Pakistan)

SARS

China, Hong

Air travel

From 1 November 2002 to 31 July 2003, SARS spread to > 25

WHO, 2003

Kong

following exposure in infected countries

countries, causing 8096 reported infections and 774 deaths.

(Continued)

Table 1.1 (Continued)

Pathogen

Site of origin

Mode of spread

Consequences

Reference

Dengue

Tahiti

Air travel;

The first autochthonous outbreak in Hawaii since 1944 occurred

Effler et al.,

association with

in 2001-2002 and caused 122 laboratory-confirmed cases.

2005

a returning

traveler with

dengue-like

symptoms

West Nile

Israel

Unknown; poss-

Since its initial detection in New York, 16,706 cases were

Hayes et al.,

Virus

ibly air travel via an infected person, bird, or mosquito

reported to the CDC between 1999 and 2004. WNV has also spread to Canada, the Caribbean, and Latin America.

2005

Influenza

Worldwide

Cruise ships

Outbreaks occurred among cruise-ship passengers between New York and Montreal, Tahiti and Hawaii, and Alaska and the Yukon Territory.

CDC, 1997; Uyeki et al., 2003

Norovirus

Worldwide

Cruise ships,

The Vessel Sanitation Program at the CDC identified > 12

Widdowson

air travel

outbreaks on cruise ships in 2002. An outbreak occurred among the crew of a flight, with limited transmission to passengers.

et al, 2004, 2005

Tuberculosis,

Worldwide,

Air travel

The 2005 TB rate in foreign-born persons in the US was 8.7

CDC, 2006c,

including

Saudi Arabia

times that in US-born persons; the incidence of MDR TB is

2006d;

multi-drug

higher in low- and middle-income countries. Comparison of TB

Gushulak and

resistant

tests using a whole-blood assay (Quanti-FERON TB assay) prior

MacPherson,

tuberculosis

to and after return from the Hajj showed 10% conversion consistent with exposure during the pilgrimage.

2004; WilderSmith et al 2005

Mumps

UK

Air travel

Summer-camp outbreak in New York involved 31 cases and was associated with a counselor from the UK; attack rate was 5.7%. Outbreak began in Iowa in December 2005, and 2597 cases were reported from 11 states between 1 January and 2 May 2006; some cases were potentially infectious during air travel.

CDC, 2006e, 2006f, 2006g

Meningo

Saudi Arabia

Air travel

Pilgrims to the Hajj became infected and spread strains into

Moore et al.,

coccal

other regions.

1989; CDC

meningitis

2000, 2001; Wilder-Smith et al, 2002; Dull et al, 2005

The development of the travel industry has also altered the landscape in many countries, where luxury hotels occupy prime real estate and new infrastructure is created to serve travelers. Although much travel tends to follow well-trodden paths to well-known places, increasingly travelers are seeking remote locations -and today's transportation technology makes it increasingly easier than ever before to reach these areas.

All of this massive movement of the human population and trade is occurring as an overlay on the background of the natural migration of animals. Although birds, land animals, and marine animals can move thousands of kilometers in seasonal migrations, humans today have access to more parts of the Earth than any other species. Humans have also altered the planet (land, sea, and atmosphere) more than any other species.

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